There is a reason why every bike mechanic I know will tell you that WD40 is a poor chain lubricant, myself included.

Wet lubes attract dust so a lubed chain is going to get dirty and require periodic cleaning and this is the nature of chain drives... dry lubes are great if you live in places where it does not rain much and during the mid summer here, dry lube is great stuff.

If the drive is all shiny and clean because you are using WD40 it is because there isn't any lube there to pick up dust and dirt so of course it will stay cleaner.. I have a lot of people tell me that is why they were using WD40 in the first place... as I tell them they need to replace their prematurely worn chains (and drive components) and then explain why they need to use the proper lube.

Here's a better test.

Take a new chain (a cheap one) and break it in half.

Lube one half with WD40 and make sure it washes out all the factory lube and if it is a pre-lubed chain just leave the other half. If not, lube it with some home brew (great stuff btw)

Hang them outside for a few weeks, if it doesn't rain spray them with a hose a few times to simulate some good rain.

If you are really confident that the WD40 is adequate, use a Campy 11 speed chain for the WD40 lubricated chain.

The geek in me wants to hook up a drive motor that would run two parallel drives with a Wd40 lubed chain and a properly oiled chain and then run it to the point of chain failure.

But really... I have been turning wrenches for a long time and see what happens to chains when they are improperly lubricated and have heard the "I thouht WD40 was a good lubricant" way too many times.